Digital multimeter M832. Electric scheme, description, characteristics. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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It's impossible to imagine a repairman's desktop without a handy inexpensive digital multimeter. This article discusses the design of the 830 series digital multimeters, the most common malfunctions and how to resolve them.

Currently, a huge variety of digital measuring instruments of varying degrees of complexity, reliability and quality is being produced. The basis of all modern digital multimeters is an integrated analog-to-digital voltage converter (ADC). One of the first such ADCs suitable for building inexpensive portable measuring instruments was the ICL71O6 chip converter manufactured by MAXIM. As a result, several successful inexpensive models of digital multimeters of the 830th series were developed, such as M830B, M830, M832, M838. Instead of the letter M, DT can stand. Currently, this series of devices is the most widespread and most repeated in the world. Its basic features: measurement of direct and alternating voltages up to 1000 V (input resistance 1 MΩ), measurement of direct currents up to 10 A, measurement of resistances up to 2 MΩ, testing of diodes and transistors. In addition, in some models there is a mode of sound continuity of connections, temperature measurement with and without a thermocouple, generation of a meander with a frequency of 50 ... 60 Hz or 1 kHz. The main manufacturer of this series of multimeters is Precision Mastech Enterprises (Hong Kong).

Scheme and operation of the device

Rice. 1. Structural diagram of ADC 7106 (click to enlarge)

The basis of the multimeter is ADC IC1 type 7106 (the closest domestic analogue is the 572PV5 microcircuit). Its block diagram is shown in fig. 1, and the pinout for execution in the DIP-40 package - in fig. 2. The 7106 kernel may have different prefixes depending on the manufacturer: ICL7106, TC7106, etc. Recently, unpackaged microcircuits (DIE chips) have been increasingly used, the crystal of which is soldered directly to the printed circuit board.

Rice. 2. ADC 7106 pinout in DIP-40 package

Consider the circuit of the M832 multimeter from Mastech (Fig. 3). Pin 1 of IC1 has a positive 9V battery supply voltage, pin 26 is negative. Inside the ADC there is a stabilized voltage source of 3 V, its input is connected to pin 1 of IC1, and the output is connected to pin 32. Pin 32 is connected to the common terminal of the multimeter and is galvanically connected to the COM input of the device.

The voltage difference between terminals 1 and 32 is approximately 3 V in a wide range of supply voltages - from nominal to 6,5 V. This stabilized voltage is supplied to the adjustable divider R11, VR1, R13, and its output is to the input of microcircuit 36 ​​(in measurement mode currents and voltages).

The divider sets the potential U eg at pin 36, equal to 100 mV. Resistors R12, R25 and R26 perform protective functions. Transistor Q102 and resistors R109, R110nR111 are responsible for low battery indication. Capacitors C7, C8 and resistors R19, R20 are responsible for displaying the decimal points of the display.

Rice. 3. Schematic diagram of the M832 multimeter (click to enlarge)

The operating input voltage range Umax directly depends on the level of the adjustable reference voltage at terminals 36 and 35 and is:

The stability and accuracy of the display reading depends on the stability of this voltage reference. The display reading N depends on the input voltage UBX and is expressed as a number:

Consider the operation of the device in the main modes.

Measuring voltage

A simplified diagram of the multimeter in voltage measurement mode is shown in fig. 4. When measuring direct voltage, the input signal is applied to R1 ... R6, from the output of which, through a switch (according to the scheme 1-8 / 1 ... 1-8 / 2), it is fed to the protective resistor R17. This resistor also forms a low-pass filter together with capacitor C3 when measuring AC voltage. Next, the signal is fed to the direct input of the ADC chip, pin 31. The potential of the common output generated by a stabilized voltage source of 3 V, pin 32 is applied to the inverse input of the microcircuit.

Rice. 4. Simplified diagram of a multimeter in voltage measurement mode

When measuring AC voltage, it is rectified by a half-wave rectifier on diode D1. Resistors R1 and R2 are selected in such a way that when measuring a sinusoidal voltage, the device shows the correct value. ADC protection is provided by R1...R6 divider and R17 resistor.

Current measurement

Rice. 5. Simplified diagram of a multimeter in current measurement mode

A simplified diagram of the multimeter in the current measurement mode is shown in fig. 5. In the DC measurement mode, the latter flows through the resistors RO, R8, R7 and R6, switched depending on the measurement range. The voltage drop across these resistors through R17 is fed to the input of the ADC, and the result is displayed. ADC protection is provided by diodes D2, D3 (may not be installed in some models) and fuse F.

Resistance measurement

Rice. 6. Simplified diagram of a multimeter in resistance measurement mode

A simplified diagram of the multimeter in the resistance measurement mode is shown in fig. 6. In the resistance measurement mode, the dependence expressed by the formula (2) is used. The diagram shows that the same current from the voltage source +LJ flows through the reference resistor Ron and the measured resistor Rx (the currents of inputs 35, 36, 30 and 31 are negligible) and the ratio of UBX and Uon is equal to the ratio of the resistances of resistors Rx and Ron. R1 .... R6 are used as reference resistors, R10 and R103 are used as current-setting resistors. Protection of the ADC is provided by the thermistor R18 [some cheap models use conventional resistors with a nominal value of 1 ... 2 kOhm), transistor Q1 in zener diode mode (not always installed) and resistors R35, R16 and R17 at inputs 36, 35 and 31 of the ADC.

Call mode

The continuity circuit uses an IC2 chip (LM358) containing two operational amplifiers. A sound generator is assembled on one amplifier, a comparator on the other. When the voltage at the input of the comparator (pin 6) is less than the threshold, a low voltage is set at its output (pin 7), which opens the key on transistor Q101, as a result of which an audible signal sounds. The threshold is determined by the divider R103, R104. Protection is provided by resistor R106 at the input of the comparator.

Multimeter Defects

All malfunctions can be divided into factory defects (and this happens) and damage caused by erroneous actions of the operator.

Since multimeters use dense mounting, element short circuits, poor soldering and breakage of element leads, especially those located along the edges of the board, are possible. Repair of a faulty device should begin with a visual inspection of the printed circuit board. The most common factory defects of M832 multimeters are shown in the table.

The serviceability of the LCD display can be checked using an alternating voltage source with a frequency of 50 ... 60 Hz and an amplitude of several volts. As such an AC voltage source, you can take the M832 multimeter, which has a meander generation mode. To check the display, put it on a flat surface with the display up, connect one M832 multimeter probe to the common output of the indicator (bottom row, left output), and apply the other multimeter probe alternately to the rest of the display outputs. If you can get the ignition of all segments of the display, then it is working.

The above malfunctions may also appear during operation. It should be noted that in the DC voltage measurement mode, the device rarely fails, because. well protected from input overloads. The main problems arise when measuring current or resistance.

Repair of a faulty device should begin with checking the supply voltage and the ADC operability: the stabilization voltage is 3 V and the absence of a breakdown between the power outputs and the common output of the ADC.

In the current measurement mode when using the V, Ω and mA inputs, despite the presence of a fuse, there may be cases when the fuse burns out later than the safety diodes D2 or D3 have time to break through. If a fuse is installed in the multimeter that does not meet the requirements of the instructions, then in this case, the resistances R5 ... R8 may burn out, and this may not appear visually on the resistances. In the first case, when only the diode breaks through, the defect appears only in the current measurement mode: the current flows through the device, but the display shows zeros. In the event of burnout of resistors R5 or R6 in the voltage measurement mode, the device will overestimate the readings or show an overload. When one or both resistors are completely burned out, the device is not reset in the voltage measurement mode, but when the inputs are closed, the display is set to zero. When resistors R7 or R8 burn out on the current measurement ranges of 20 mA and 200 mA, the device will show an overload, and in the range of 10 A - only zeros.

In resistance measurement mode, faults typically occur in the 200 ohm and 2000 ohm ranges. In this case, when voltage is applied to the input, resistors R5, R6, R10, R18, transistor Q1 can burn out and capacitor Sat breaks through. If transistor Q1 is completely broken, then when measuring resistance, the device will show zeros. With an incomplete breakdown of the transistor, the multimeter with open probes will show the resistance of this transistor. In the voltage and current measurement modes, the transistor is short-circuited by the switch and does not affect the multimeter readings. In the event of a breakdown of capacitor C6, the multimeter will not measure voltage in the ranges of 20 V, 200 V and 1000 V or significantly underestimate the readings in these ranges.

If there is no indication on the display when there is power to the ADC, or if a large number of circuit elements are visually burnt out, there is a high probability of damage to the ADC. The serviceability of the ADC is checked by monitoring the voltage of a stabilized voltage source of 3 V. In practice, the ADC burns out only when a high voltage is applied to the input, much higher than 220 V. Very often, cracks appear in the frameless ADC compound, the current consumption of the microcircuit increases, which leads to its noticeable heating .

When a very high voltage is applied to the input of the device in the voltage measurement mode, a breakdown may occur along the elements (resistors) and along the printed circuit board; in the case of the voltage measurement mode, the circuit is protected by a divider on resistances R1 ... R6.

For cheap models of the DT series, long leads of parts can be shorted to the screen located on the back of the device, disrupting the operation of the circuit. Mastech does not have such defects.

A stabilized voltage source of 3 V in the ADC for cheap Chinese models can in practice give a voltage of 2,6 ... 3,4 V, and for some devices it stops working already at a supply battery voltage of 8,5 V.

The DT models use low quality ADCs and are very sensitive to the C4 and R14 integrator circuit values. In Mastech multimeters, high-quality ADCs make it possible to use elements of close ratings.

Often in DT multimeters with open probes in the resistance measurement mode, the device approaches the overload value for a very long time ("1" on the display) or is not set at all. You can "cure" a low-quality ADC chip by reducing the value of the resistance R14 from 300 to 100 kOhm.

When measuring resistances in the upper part of the range, the device "fills up" the readings, for example, when measuring a resistor with a resistance of 19,8 kOhm, it shows 19,3 kOhm. It is "treated" by replacing the capacitor C4 with a capacitor of 0,22 ... 0,27 microfarads.

Since cheap Chinese firms use low-quality frameless ADCs, there are often cases of broken outputs, while it is very difficult to determine the cause of the malfunction and it can manifest itself in different ways, depending on the broken output. For example, one of the indicator outputs is not lit. Since multimeters use displays with static indication, in order to determine the cause of the malfunction, it is necessary to check the voltage at the corresponding output of the ADC chip, it should be about 0,5 V relative to the common output. If it is zero, then the ADC is faulty.

An effective way to find the cause of a malfunction is to check the outputs of the analog-to-digital converter chip as follows. Another, of course, serviceable, digital multimeter is used. It enters the diode test mode. The black probe, as usual, is installed in the COM jack, and the red one in the VQmA jack. The red probe of the device is connected to pin 26 [minus power), and the black one touches each leg of the ADC chip in turn. Since protective diodes in reverse connection are installed at the inputs of the analog-to-digital converter, with this connection they should open, which will be reflected on the display as a voltage drop across the open diode. The actual value of this voltage on the display will be slightly higher, because. resistors are included in the circuit. In the same way, all the ADC outputs are checked when the black probe is connected to pin 1 [to the ADC power plus) and alternately touching the remaining outputs of the microcircuit. The instrument readings should be similar. But if you change the polarity of the inclusion during these checks to the opposite, then the device should always show an open circuit, because. the input impedance of a good chip is very high. Thus, outputs that show finite resistance for any polarity of connection to the microcircuit can be considered faulty. If the device shows a break with any connection of the output under study, then this ninety percent indicates an internal break. This verification method is quite universal and can be used when testing various digital and analog microcircuits.

There are malfunctions associated with poor-quality contacts on the biscuit switch, the device only works when the biscuit is pressed. Companies that produce cheap multimeters rarely cover the tracks under the biscuit switch with grease, which is why they quickly oxidize. Often the paths are dirty with something. It is repaired as follows: the printed circuit board is removed from the case, and the switch tracks are wiped with alcohol. Then a thin layer of technical petroleum jelly is applied. Everything, the device is repaired.

With devices of the DT series, it sometimes happens that the alternating voltage is measured with a minus sign. This indicates that D1 has been installed incorrectly, usually due to incorrect markings on the body of the diode.

It happens that manufacturers of cheap multimeters put low-quality operational amplifiers in the sound generator circuit, and then when the device is turned on, the buzzer buzzes. This defect is eliminated by soldering an electrolytic capacitor with a nominal value of 5 microfarads in parallel with the power circuit. If this does not ensure stable operation of the sound generator, then it is necessary to replace the operational amplifier with an LM358P.

Often there is such a nuisance as battery leakage. Small drops of electrolyte can be wiped with alcohol, but if the board is heavily flooded, then good results can be obtained by washing it with hot water and laundry soap. After removing the indicator and unsoldering the squeaker, using a brush, such as a toothbrush, you need to carefully lather the board on both sides and rinse it under running tap water. After washing 2...3 times, the board is dried and installed in the case.

In most of the devices produced recently, unpackaged (DIE chips) ADCs are used. The crystal is mounted directly on the printed circuit board and filled with resin. Unfortunately, this significantly reduces the maintainability of devices, because. when the ADC fails, which occurs quite often, it is difficult to replace it. Devices with unpackaged ADCs are sometimes sensitive to bright light. For example, when working near a table lamp, the measurement error may increase. The fact is that the indicator and the board of the device have some transparency, and the light, penetrating through them, falls on the ADC crystal, causing a photoelectric effect. To eliminate this shortcoming, you need to remove the board and, having removed the indicator, glue the location of the ADC crystal (it can be clearly seen through the board) with thick paper.

When buying DT multimeters, you should pay attention to the quality of the mechanics of the switch, be sure to turn the multimeter's switch several times to make sure that the switch occurs clearly and without jamming: plastic defects cannot be repaired.

Publication: cxem.net

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